This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as including admissions of prior art.
Modern society is greatly dependent on the use of hydrocarbons for fuels and chemical feedstocks. Hydrocarbons are generally found in subsurface rock, soil or sand formations that can be termed “reservoirs.” Removing hydrocarbons from reservoirs depends on numerous physical properties of the reservoirs, such as the permeability of the formations containing the hydrocarbons, the ability of the hydrocarbons to flow through the formations, and/or the proportion of hydrocarbons present, among other things.
Easily produced sources of hydrocarbons are dwindling, leaving less conventional sources to satisfy future energy needs. As the costs of hydrocarbons increase, less conventional sources become economically attractive to produce. For example, the production of oil sands has become more economical. The hydrocarbons produced from less conventional sources may have relatively high viscosities, for example, ranging from 1000 centipoise to 20 million centipoise with API (American Petroleum Institute) densities ranging from 8° API, or lower, up to 20° API, or higher. The hydrocarbons harvested from less conventional sources may include bitumen, or other carbonaceous materials, collectively referred to herein as “heavy oil.” The hydrocarbons produced from less conventional sources are difficult to recover using conventional techniques.
Several methods have been developed to recover heavy oil from, for example, oil sands. Strip or surface mining may be performed to access oil sands. Once accessed, the oil sands may be treated with hot water or steam to recover the heavy oil. For formations where heavy oil is not close to the Earth's surface, heat may be added and/or dilution may be used to reduce the viscosity of the heavy oil and recover the heavy oil. Heat may be supplied through a heating agent like steam. The recovered heavy oil may or may not be produced via a production well or wellbore. The production well or wellbore may be the same as the wellbore used to inject the heat for the steam injection. If the heating agent is steam, the steam may condense to water at the steam/cooler-oil-sands (SCO) interface in the formation and supply latent heat of condensation to heat the heavy oil in the oil sands, thereby reducing viscosity of the heavy oil and causing the heavy oil to flow more easily.
A number of steam-based heavy oil processes have been developed for recovering heavy oil. The processes may include, for example, cyclic steam stimulation (CSS), steam flooding, steam-assisted gravity drainage (SAGD), and solvent-assisted steam-assisted gravity drainage (SA-SAGD).
CSS may or may not raise a steam injection pressure above a formation fracturing pressure to create fractures within the formation and enhance a surface area to allow access of the steam to the heavy oil. Steam may increase a temperature of the heavy oil during a heat-soak phase, thereby lowering a viscosity of the heavy oil. The injection well may be used to produce the heavy oil. The cycle of soalking and producing may be repeated until the cost of injecting steam becomes uneconomical. For instance, steam from successive steam injection cycles may re-enter earlier created fractures and, thus, the process becomes less efficient over time. CSS may be practiced in horizontal or vertical wells. CSS processes are described in U.S. Pat. No. 3,292,702, and U.S. Pat. No. 3,739,852, among others.
Steam flooding is a process in which steam is injected from a series of vertical well injectors or horizontal well injectors and heavy oil is heated and pushed towards a series of vertical producer wells or horizontal producer wells. Steam flooding can be used as a late life process after a CSS operation. Steam flooding in late life is essentially a gravity drainage process. Solvent can be injected with steam to enhance the steam flooding. Further details may be obtained, for example, from Zhihong Liu and Shane D. Stark, “Reservoir Simulation Modelling of the Mature Cold Lake Steaming Operations,” Society of Petroleum Engineers, SPE 160491, presented in Calgary, Alberta, 12-14 Jun., 2012.
SAGD is a process where two horizontal wells may be completed in the reservoir. The two wells may be first drilled vertically to different depths within the reservoir. Thereafter, using directional drilling technology, the two wells may be extended in the horizontal direction that results in two horizontal wells, each vertically spaced from, but otherwise vertically aligned with, the other. Ideally, the production well may be located above the base of the reservoir but as close as practical to the base of the reservoir, and the injection well may be located vertically 10 to 30 feet (3 to 10 meters) above the horizontal production well. The upper horizontal well may be utilized as an injection well and may be supplied with steam from the surface. The steam may rise from the injection well, permeating through the reservoir to form a vapor chamber (steam chamber). As the vapor chamber grows over time towards the top of the reservoir, the steam may condense at the SCO interface, releasing latent heat of steam and, thereby reducing the viscosity of the heavy oil in the reservoir. The heavy oil and condensed steam may then drain downward through the reservoir under the action of gravity and flow into the lower production well, from where the heavy oil and condensed steam can be pumped to the surface. At the surface of the well, the condensed steam and heavy oil may be separated, and the heavy oil may be diluted with appropriate light hydrocarbons for transportation by pipeline. SAGD processes are described in Canadian Patent No. 1,304,287 and in U.S. Pat. No. 4,344,485.
In steam-based heavy oil processes, the water from condensed steam and the heavy oil may compete with each other while flowing through the pore space of the reservoir to reach the production well. A high interfacial tension (IFT) between water and heavy oil may adversely affect the heavy oil recovery. IFT reducing agents have been proposed to be injected with the steam. Such IFT reducing agents are often high boiling liquids that do not vaporize and therefore do not travel with steam easily and condense where they need to, i.e. at the SCO interface, for the greatest effect.
Heavy oil diluting agents may be injected into the formation with steam to vaporize and condense with the steam at the SCO interface to reduce the viscosity of the heavy oil by dilution. Methods employing heavy oil diluting agents in combination with steam are commonly referred to as SA-SAGD. Examples of such heavy oil diluting agents are individual alkanes, mixtures of alkanes, and gas plant condensates (mixtures of alkanes, aromatic hydrocarbons, and naphthenes). Such diluting agents lack IFT reducing capability and have lower latent heats of condensation than steam. As such, the heavy oil diluting agents do not deliver as much heat to the SCO interface upon condensation. The heavy oil diluting agents also are deficient in suitable Hansen Solubility Parameters that can be used to assess if one material will dissolve in another and form a solution.
Heavy oil diluting agents and IFT reducing agents have been regarded as two distinct species of compounds in the past and have been used independently for their desired effects. For example, Canadian Patent No. 2,323,029 discloses the use of heavy oil diluting agents with the steam. Suitable additives are said to be C1 to C25 hydrocarbons and combinations thereof. While such additives may have an ability to dilute heavy oils, they do not have significant IFT reducing ability. Canadian Patent No. 2,342,955 discloses a laser-based CSS process (LASER CSS) that involves mixing liquid hydrocarbons into the injected steam, but the heavy oil diluting agents have little IFT reducing ability.
The teachings of all of the patent and non-patent documents identified and referred to above are specifically incorporated into this disclosure by reference.
There is a need for improved or alternative agents to employ with steam-based thermal recovery processes.